Optical sensors have been used for oil condition monitoring for determining the presence of debris or otherwise monitor deterioration of a lubricant. Such devices may operate by shining light through a small gap and analyzing the transmitted light with a suitable optical sensor. Alternative sensors may make use of scattering of light and may operate over different frequencies including outside of the visible range. Oil condition monitoring may be significant in providing feedback in advance of likely failure of a lubricant system. Action may be taken to perform maintenance or otherwise renew the lubricant.
Water in oil is of considerable concern to many mechanical systems. Minimal amounts of water may be absorbed by the oil during use, either from the atmosphere or by direct ingress of water into the system. As long as this water is in the absorbed state and the oil is unsaturated, the concern is minimal. Nevertheless, as the concentration of water approaches the saturation level, emulsified and free water may occur, which can be highly detrimental, especially if exposure is prolonged. In bearings, the incompressibility of water relative to the oil can result in disruption of the oil film leading to excessive wear. Just one percent water in oil can reduce the life expectancy of a bearing by as much as 90 percent. For ball or rolling element bearings, the localized pressure generated can cause spontaneous vaporization of the water, leading to erosive wear such as micropitting. The saturation level of water in oil may vary widely according to temperature and the type of oil and can range from 10 ppm to even 10000 ppm. Existing sensors capable of measuring the presence of water (free and dissolved) include capacitive sensors and Karl Fischer titration sensors. Both of these methods require considerable time for the sensor to reach equilibrium and are not ideal for rapidly changing conditions. Spectral analysis using Fourier Transform Infrared Spectroscopy (FTIR) has been used but is a relatively complex and costly procedure requiring calibration of the sensor relative to the spectrum produced with fresh oil. Spectral analyzers are also relatively costly, bulky and sensitive devices for installation in many environments where mechanical systems are located.
It would thus be desirable to provide for a low-cost and simple sensor arrangement that could reliably identify the presence of water in oil in real time.